subscribe method Null safety

Push subscribe(
  1. {Duration? timeout}
)

Implementation

Push subscribe({Duration? timeout}) {
  if (_joinedOnce == true) {
    throw "tried to subscribe multiple times. 'subscribe' can only be called a single time per channel instance";
  } else {
    _joinedOnce = true;
    rejoin(timeout ?? _timeout);
    return _joinPush;
  }
}